SN 2024gy: Multiepoch Spectroscopic Features Suggestive of Delayed Detonation in a Type Ia Supernova

Abstract We present photometric and spectroscopic observations of SN 2024gy, a Type Ia supernova (SN Ia) exhibiting high-velocity features (HVFs) in its early-time spectra. This SN reaches a peak B -band magnitude of −19.25 ± 0.29 mag and subsequently declines by Δ m 15 ( B ) ≈ 1.12 mag, consistent with the luminosity–width relation characteristic of normal SNe Ia. Based on the peak thermal luminosity of (1.2 ± 0.3) × 10 43 erg s −1 , we estimate that 0.57 ± 0.14 M ⊙ of 56 Ni was synthesized during the explosion. Our dense early spectral monitoring revealed significant velocity disparities within the ejecta. Notably, absorption features from the Ca ii near-infrared triplet were observed at velocities exceeding 25,000 km s −1 , while the Si ii λ 6355 line velocity at the same epoch was significantly lower at ∼16,000 km s −1 . This velocity disparity likely reflects distinct ionization states of intermediate-mass elements in the outermost layers. The prominent Ca ii HVFs may originate from ionization suppression within the highest-velocity ejecta, potentially indicative of minimal hydrogen mixing in a delayed-detonation explosion scenario. Additionally, the Ni/Fe ratio derived from the nebular spectrum of SN 2024gy provides further support for this model.